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Tsuda, Takashi; Kurita, Genichi; Fujita, Takaaki
Journal of Plasma Physics, 72(6), p.1149 - 1152, 2006/12
Times Cited Count：3 Percentile：10.78(Physics, Fluids & Plasmas)The current density is expected to be negative in the central region of the tokamak when the amplitude of bootstrap current or off-axis current drive is large enough and a negative one-turn voltage exists. However, a flat current profile with almost zero value has been observed in experiments. The current profile with Current Hole has a tendency to be unstable to double tearing mode and we investigate the role of double tearing mode on the formation of Current Hole with RMHD simulation.
Tsuda, Takashi; Kurita, Genichi; Fujita, Takaaki
Journal of the Korean Physical Society, 49, p.S83 - S86, 2006/12
The current density is expected to be negative in the central region of the tokamak when the amplitude of bootstrap current or off-axis current drive is large enough. However, a flat current profile with almost zero value has been observed in experiments even under the situation with negative one-turn voltage exists in the central region of plasma. Double tearing mode (DTM) can be unstable for the current profile with a "current hole" and some MHD activities are observed in JET before the formation of the current hole. On the contrary, no MHD activity is observed in the JT-60 experiment. Here, we study the condition of appearance of DTM and investigate the stability of DTM and the interaction between DTM (n=1 perturbation) and "Current Hole" with resistive RMHD simulations.
Konovalov, S. V.; Mikhailovskii, A. B.*; Ozeki, Takahisa; Takizuka, Tomonori; Shirokov, M. S.*; Hayashi, Nobuhiko
Plasma Physics and Controlled Fusion, 47(12B), p.B223 - B236, 2005/12
Times Cited Count：7 Percentile：24.62(Physics, Fluids & Plasmas)Neoclassical Tearing Modes, NTMs, are known to limit the maximum attainable plasma pressure, degrade confinement and, at worst, culminate in disruption of the tokamak discharges. Key role in NTM evolution belongs to the radial profiles of the plasma flow, temperature and density which are determined by the conjunction of the longitudinal and cross-filed transport arising from thermal conduction, particle diffusion, and viscosity. In this report the influence of anomalous perpendicular heat transport and anomalous ion perpendicular viscosity on early stage of the NTM evolution is studied theoretically.
Matsumoto, Taro; Naito, Hiroshi*; Tokuda, Shinji; Kishimoto, Yasuaki*
Nuclear Fusion, 45(11), p.1264 - 1270, 2005/11
Times Cited Count：14 Percentile：43.63(Physics, Fluids & Plasmas)A gyrokinetic particle simulation is executed to clarify the effect of the electron inertia on the MHD phenomena in the reversed shear configuration (RSC) of a cylindrical tokamak plasma. It is found that the collisionless (kinetic) double tearing modes grow up at the Alfv
n time scale, and nonlinearly induce the internal collapse when the helical flux at the magnetic axis is less than that at the outer resonant surface. After the internal collapse, the secondary reconnection is induced by the current concentration due to the 
convective flow. It is also clarified that a nonlinear dynamics accompanied with the elementary processes caused by the flow can generate a new RSC with resonant surfaces. In the presence of the density gradient, after the full reconnection induced by the mode, the radial electric field is found to be generated due to the difference of the 
motion between ions and electrons. However, the intensity of the radial field is not so large as that induced by the collisionless kink mode.
Suzuki, Takahiro; Isayama, Akihiko; Ide, Shunsuke; Fujita, Takaaki; Oikawa, Toshihiro; Sakata, Shinya; Sueoka, Michiharu; Hosoyama, Hiroki*; Seki, Masami; JT-60 Team
AIP Conference Proceedings 787, p.279 - 286, 2005/09
A real-time control system of safety factor (q) profile was developed in JT-60. This system, for the first time, enables 1) real time evaluation of q profile using local magnetic pitch angle measurement by motional Stark effect (MSE) diagnostic and 2) control of current drive (CD) location (rhoCD) by adjusting the parallel refractive index 
of lower-hybrid (LH) waves through the change of phase difference (dphi) of LH waves between multi-junction launcher modules. The method for q profile evaluation was newly developed, without time-consuming reconstruction of equilibrium, so that the method requires less computational time. The system evaluates q profile within every 10ms, which is much faster than current relaxation time, typically order of 1s. Safety factor profile by the real-time calculation agreed well with that by equilibrium reconstruction with MSE. From temporal evolution of q (or current) profile, we evaluate CD location in real-time, too. The control system controls rhoCD through (or directly dphi) in such a way to minimize difference between the real-time evaluated q profile and its reference profile. The real-time control system was applied to positive shear plasmas (
), having plasma current of 0.6MA, toroidal field of 2.3T, and electron density of 
. In order to keep good coupling of LH waves to the plasma, gap between the launcher and the plasma surface was controlled to about 0.1m. The reference q profile was set to q(0)=1.3. The real-time q profile approached to the reference after application of real-time control; the controlled q profile was sustained for 3s, which was limited by injected LH power. RF experiments in JT-60U, such as stabilization of neo-classical tearing modes, plasma startup experiments, etc., are also presented.
Oikawa, Toshihiro; Suzuki, Takahiro; Isayama, Akihiko; Hayashi, Nobuhiko; Fujita, Takaaki; Naito, Osamu; Tsuda, Takashi; Kurita, Genichi; JT-60 Team
Nuclear Fusion, 45(9), p.1101 - 1108, 2005/09
Times Cited Count：4 Percentile：14.24(Physics, Fluids & Plasmas)Evolution of the current density profile associated with magnetic island formation in a neoclassical tearing mode plasma was measured for the first time in JT-60U by. As the island grew, the current density profile turned flat at the radial region of the island, followed by an appearance of a hollow structure. As the island shrank, the deformed region became narrower, and it finally diminished after the disappearance of the island. In an MHD-quiescent plasma, on the other hand, no deformation was observed. The observed deformation in the current density profile associated with the tearing mode is reproduced in a time dependent transport simulation assuming reduction of the bootstrap current in the radial region of the island. Comparison of the measurement with a calculated steady-state solution also explains the temporal behaviors of the current density and safety factor profiles with reduction and recovery of the bootstrap current. From the experimental observation and simulations, we reach conclusion that the bootstrap current decreases within the island O-point.
Matsumoto, Taro; Naito, Hiroshi*; Tokuda, Shinji; Kishimoto, Yasuaki
Physics of Plasmas, 12(9), p.092505_1 - 092505_7, 2005/09
Times Cited Count：2 Percentile：6.86(Physics, Fluids & Plasmas)The behavior of the collisionless magnetohydrodynamics modes is investigated by the gyro-kinetic particle simulation in a cylindrical tokamak plasma in the parameter region where the effects of electron inertia and electron parallel compressibility are competitive for magnetic reconnection. Although the linear growth of the 
internal kink-tearing mode is dominated by the electron inertia, it is found that the growth rate can be nonlinearly accelerated due to the electron parallel compressibility proportional to the ion sound Larmor radius 
. It is also found that, as decreasing the electron skin depth 
, the maximum growth rate before the internal collapse saturates independently of the microscopic scales such as and . The acceleration of growth rate is also observed in the nonlinear phase of the double tearing mode.
Oikawa, Toshihiro; Isayama, Akihiko; Fujita, Takaaki; Suzuki, Takahiro; Tsuda, Takashi; Kurita, Genichi
Physical Review Letters, 94(12), p.125003_1 - 125003_4, 2005/04
Times Cited Count：10 Percentile：53.46(Physics, Multidisciplinary)Evolution of the current density profile associated with the magnetic island formation during a tearing mode was measured for the first time in the JT-60U tokamak. With the island growth, the current density profile turned flat at the radial region of the island, followed by appearance of a hollow structure. As the island shrank, the flat region became narrower, and it finally diminished after disappearance of the island. It was also observed that the local poloidal magnetic field fluctuated in correlation with the island rotation. This indicates that the observed deformation in the current density profile is localized in the O-point of the island. The result is the first experimental demonstration of theoretical predictions on the current density evolution in tearing modes.
Konovalov, S. V.; Mikhailovskii, A.*; Ozeki, Takahisa; Takizuka, Tomonori; Shirokov, M. S.*; Hayashi, Nobuhiko
Proceedings of 20th IAEA Fusion Energy Conference (FEC 2004) (CD-ROM), 8 Pages, 2004/11
Influence of anomalous perpendicular heat transport and anomalous perpendicular viscosity on conditions of Neoclassical Tearing Mode onset is studied theoretically. Series of different parallel transport mechanisms competitive to anomalous cross-island heat conductivity in the perturbed temperature profile are considered.In the transport threshold model, magnetic curvature effect is as much important as bootstrap drive weakened by the finite perpendicular heat transport. Perpendicular viscosity brings island-rotation-frequency dependence of the transport threshold model.
Hayashi, Nobuhiko; Isayama, Akihiko; Nagasaki, Kazunobu*; Ozeki, Takahisa
Purazuma, Kaku Yugo Gakkai-Shi, 80(7), p.605 - 613, 2004/07
Neoclassical tearing mode stabilization by an electron cyclotron wave current drive (ECCD) has been studied by using the numerical model on the basis on the modified Rutherford eq. coupled with the 1.5D transport code and the EC code. Numerical model almost reproduces the JT-60U experimental results. Undetermined parameters in the modified Rutherford equation are estimated from JT-60U experiments. Sensitivity of stabilization to the EC current location is investigated. Low EC current and peaked EC current profile mitigates the sensitivity, while higher EC current and the peaked EC current profile moves the rational surface more largely through the background current modification by ECCD and intensify the sensitivity. High EC current and broad EC current profile also mitigates the sensitivity. EC current necessary for the full stabilization is studied. The necessary EC current much depends on the parameters of bootstrap current and ECCD terms in the modified Rutherford equation. Necessary ECCD power on ITER is evaluated on the basis of parameters estimated from JT-60U experiments.
Hayashi, Nobuhiko; Ozeki, Takahisa; Hamamatsu, Kiyotaka; Takizuka, Tomonori
Nuclear Fusion, 44(4), p.477 - 487, 2004/04
Times Cited Count：35 Percentile：72.74(Physics, Fluids & Plasmas)NTM stabilization by ECCD has been numerically studied in order to evaluate the necessary EC power for the stabilization of NTM on ITER. The time evolution of an island width of NTM is calculated by the modified Rutherford equation. The modification of background current profile by the EC current is calculated by the current diffusion equation and the variation of the tearing stability index 
' is taken into account. When the EC power is higher than a threshold value, NTM with any island width can be fully stabilized. The dependence of the threshold power on parameters in the modified Rutherford equation is examined. The threshold power much depends on the parameters of the bootstrap current term in the modified Rutherford equation. The injected EC current decreases the value of ' through the background current modification, which results in the reduction of the threshold power. The effects of the peakedness of EC current profile and the EC power modulation on the threshold power are investigated. When the width of the EC current profile becomes half, the threshold power is reduced to half or less. The EC power modulation is inessential for the threshold power reduction if the EC current profile can be peaked. Considering the maximum value of the threshold power in the range of the parameters, the EC power of about 25 MW is found to be sufficient for the simultaneous stabilization of both the 
=3/2 and 2/1 mode NTM on ITER.
Mukhovatov, V.*; Shimada, Michiya; Chudnovskiy, A. N.*; Costley, A. E.*; Gribov, Y.*; Federici, G.*; Kardaun, O. J. F.*; Kukushkin, A. S.*; Polevoi, A. R.*; Pustovitov, V. D.*; et al.
Plasma Physics and Controlled Fusion, 45(12), p.235 - 252, 2003/12
Times Cited Count：55 Percentile：82.78(Physics, Fluids & Plasmas)ITER will be the first magnetic confinement device with burning DT plasma and fusion power of about 0.5 GW. During the past few years, new results have been obtained that substantiate the confidence in achieving Q 
10 in ITER with inductive H-mode operation. These include achievement of a good H-mode confinement near the Greenwald density at high triangularity of the plasma cross section; improvements in theory-based confinement projections for the core plasma; improvement in helium ash removal due to the elastic collisions of He atoms with D/T ions in the divertor predicted by modelling; demonstration of feedback control of NTMs and resultant improvement in the achievable beta-values; better understanding of ELM physics and development of ELM mitigation techniques; and demonstration of mitigation of plasma disruptions. ITER will have a flexibility to operate also in steady state and intermediate (hybrid) regimes. The paper concentrates on inductively driven plasma performance and discusses requirements for steady-state operation in ITER.
Mikhailovskii, A. B.*; Shirokov, M. S.*; Tsypin, V. S.*; Konovalov, S. V.; Ozeki, Takahisa; Takizuka, Tomonori; Galv
o, R. M. 0.*; Nascimento, I. C.*
Physics of Plasmas, 10(10), p.3975 - 3983, 2003/10
Times Cited Count：8 Percentile：25.85(Physics, Fluids & Plasmas)A transport threshold model of subsonic neoclassical tearing modes (NTMs) in tokamaks is developed. The basic procedure is to include the velocity-dependent term into the single-fluid heat-conductivity equation. and to supply the parallel plasma motion equation. These equations permit the determination of the perturbed plasma temperature and the bootstrap current drive of NTMs, for both strong and weak perpendicular heat transport. It is shown that the subsonic NTMs transport threshold model can be more realistic than the standard transport threshold model of NTMs.
Ishii, Yasutomo; Azumi, Masafumi; Kishimoto, Yasuaki
Physics of Plasmas, 10(9), p.3512 - 3520, 2003/09
Times Cited Count：12 Percentile：37.11(Physics, Fluids & Plasmas)no abstracts in English
Mikhailovskii, A. B.*; Shirokov, M. S.*; Tsypin, V. S.*; Konovalov, S. V.*; Ozeki, Takahisa; Takizuka, Tomonori; Galvo, R. M. 0.*; Nascimento, I. C.*
Physics of Plasmas, 10(9), p.3790 - 3792, 2003/09
Times Cited Count：5 Percentile：16.93(Physics, Fluids & Plasmas)A fluid treatment of convective-transport threshold model of neoclassical tearing modes (NTMs) in tokamaks is developed. A system of moment equations of the drift kinetic equation with a model perpendicular transport is derived. The essence of this moment equation system is to allow for the parallel heat flux on an equal footing with pressure and temperature. The suggested moment equation system is applied for analyzing the bootstrap current drive of NTMs. As a result, a threshold model of these modes is derived, which coincides qualitatively with the convective-transport threshold model initially formulated by means of intuitive consideration.
Mikhailovskii, A. B.*; Churikov, A. P.*; Konovalov, S. V.; Shirokov, M. S.*; Tsypin, V. S.*
Doklady Physics, 48(4), p.159 - 162, 2003/04
Times Cited Count：1 Percentile：12.86(Mechanics)It is shown that there is a new class of perturbations, limiting the pressure of ideally stable plasma in the conditions of fusion reactors of tokamak type. localization of such modes includes three characteristic regions: large-scale (MHD), Larmor and sub-Larmor ones. It is suggested that these modes can be responsible for spontaneous generation of neoclassical tearing modes observed experimentally.
Takeji, Satoru; Isayama, Akihiko; Ozeki, Takahisa; Tokuda, Shinji; Ishii, Yasutomo; Oikawa, Toshihiro; Ishida, Shinichi; Kamada, Yutaka; Neyatani, Yuzuru; Yoshino, Ryuji; et al.
Fusion Science and Technology (JT-60 Special Issue), 42(2-3), p.278 - 297, 2002/09
Times Cited Count：7 Percentile：6.06(Nuclear Science & Technology)no abstracts in English
Hayashi, Nobuhiko; Ozeki, Takahisa; Hamamatsu, Kiyotaka; Takizuka, Tomonori
Journal of Plasma and Fusion Research SERIES, Vol.5, p.519 - 522, 2002/00
Stabilizing simulation of the neoclassical tearing mode by an electron cyclotron current drive (ECCD) in JT-60 superconducting tokamak has been done by using a 1.5D time-dependent tokamak code TOPICS and the modified Rutherford's equation. A local EC current in the center of the island stabilizes the mode. However, the EC current off the center moves the rational surface and decreases the stabilizing effect of EC. Necessary range of EC current location for the stabilization becomes wider for low EC power, or narrower for high EC power, when the EC current profile is more peaked. The long duration of EC injection widens this range. Peaked EC current profile, early injection and long injection duration lower the minimum EC power necessary for the stabilization.
Ozeki, Takahisa; Isayama, Akihiko
Purazuma, Kaku Yugo Gakkai-Shi, 77(5), p.409 - 419, 2001/05
Neoclassical tearing modes (NTMs) which are induced by lack of the bootstrap current inside the magnetic island are reviewed based on the modified Rutherford equation. Theoretical bases and experimental observations of NTM are described. The beta value scaling of the growth of NTM and the mechanism of the growth are discussed. Influence of NTM on energy confinement is described, and the stabilization of NTM by electron cyclotron current drive (ECCD) is introduced.
Ozeki, Takahisa; Hamamatsu, Kiyotaka; Isayama, Akihiko; Kurita, Genichi
Fusion Engineering and Design, 53(1-4), p.65 - 69, 2001/01
Times Cited Count：7 Percentile：48.68(Nuclear Science & Technology)no abstracts in English
